Against the backdrop of a growing chikungunya epidemic in La Réunion, this prospective study will assess the real-life efficacy, safety and immunogenicity of IXCHIQ® vaccine in vulnerable individuals (seniors, comorbid patients), defined by the French Health Autority (HAS) as at risk of severe or complicated forms and/or chronic disabling forms (chronic arthritis, chronic fatigue phenotypes). This study will also provide input for the preparation of a cluster randomized trial on a population scale.

Inclusion Criteria:

• Eligible for anti-chikungunya vaccination according to french Health Authority (HAS)
• Beneficiary of social security coverage
• Signature of informed consent form

Exclusion Criteria:

• Pregnant women
• Vaccine contraindication for patients undergoing medical treatment
• Persons under guardianship, curatorship or safeguard of justice
• Anyone who has received immunoglobulin or a blood or plasma transfusion 3 months prior to vaccine administration